The depolarization of the atrium is used as a marker of atrial activation in the determination of atrial rate and rhythm in implantable medical devices (IMDs) such as pacemakers, implantable cardioverter defibrillators (ICDs) and implantable cardiac monitors. Typically, a cardiac electrogram (EGM) signal sensed using an electrode positioned along an atrial chamber that exceeds a programmable sensing threshold is sensed as an atrial depolarization or P-wave. Sensed P-waves are used in determining an atrial rate and detecting and discriminating abnormal heart rhythms.
Errors in determining an actual atrial rate result when atrial oversensing or atrial undersensing occurs. One cause of atrial oversensing is the presence of far-field signals exceeding the atrial sensing threshold. Far-field signals can include R-waves corresponding to the depolarization of the ventricles and other physiologic or non-physiologic noise. Far-field R-waves (FFRWs) can have an amplitude similar to P-wave amplitudes in an atrial sensed signal. The programmable atrial sensing threshold may be increased to avoid FFRW sensing however this may result in atrial P-wave undersensing. The detection of FFRWs as intrinsic atrial events can result in inappropriate atrial rate detection and confuse pattern recognition algorithms used for discriminating arrhythmias, leading to unnecessary pacing mode switching and even unnecessary arrhythmia therapy delivered by the IMD. Discrimination of near-field atrial P-waves from FFRWs during atrial flutter or atrial fibrillation can be even more challenging because the atrial P-wave amplitudes are often lower during atrial arrhythmias than during normal sinus rhythm. Reliable discrimination of near-field and far-field signals is needed in the field of cardiac sensing to enable rejection of far-field signals during monitoring of near-field signals. Discrimination of near-field and far-field signals may be useful in a variety of cardiac rhythm monitoring and therapy delivery applications and may be important in other, non-cardiac electrophysiological sensing applications as well.